Measurement apparatus and method

ABSTRACT

A measurement apparatus disclosed that has a radiation source configured to provide a measurement beam of radiation such that an individually controllable element of an array of individually controllable elements capable of modulating a beam of radiation, is illuminated by the measurement beam and redirects the measurement beam, and a detector arranged to receive the redirected measurement beam and determine the position at which the redirected measurement beam is incident upon the detector, the position at which the redirected measurement beam is incident upon the detector being indicative of a characteristic of the individually controllable element.

The present application is a continuation of U.S. patent applicationSer. No. 11/541,793, filed on Oct. 3, 2006, now U.S. Pat. No. 7,804,603,the entire contents of the foregoing application is hereby incorporatedby reference.

FIELD

The invention relates to a measurement apparatus and method. Inparticular, but not exclusively, the invention relates to a measurementapparatus and method for use in a lithographic apparatus and a devicemanufacturing method respectively.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device (e.g., a mask) may be used togenerate a circuit pattern corresponding to an individual layer of theIC, and this pattern can be imaged onto a target portion (e.g.comprising part of, one or several dies) on a substrate (e.g. a siliconwafer) that has a layer of radiation-sensitive material (resist).Instead of a mask, the patterning device may comprise a patterning arraythat comprises an array of individually controllable elements. Anadvantage of a system using a patterning array compared to a mask-basedsystem is that the pattern can be changed more quickly and for lesscost.

In general, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion in one go, andso-called scanners, in which each target portion is irradiated byscanning the pattern through the beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction.

A lithographic apparatus typically comprises an illuminator configuredto provide a conditioned illumination beam of radiation. In order toprovide a shaped illumination beam, one or more masks may be providedwithin the illuminator in order to block off portions of theillumination beam, thus changing the pupil shape of the illuminationbeam. Alternatively or additionally, an array of individuallycontrollable elements, such as a programmable mirror array, arranged toselectively reflect portions of the illumination beam may be provided inorder to create a shaped illumination beam that can be controlled, andthus readily changed from one cross sectional pattern to another.However, the illumination beam typically operates at a relatively highintensity, such that, for example, the array of individuallycontrollable elements may become heated. Such heating may, for example,affect the accuracy of the beam path along which the illumination beamis selectively reflected, thus causing the shaped illumination beam todeviate from the expected shape.

SUMMARY

According to an aspect of the invention, there is provided a measurementapparatus comprising:

-   -   a radiation source configured to provide a measurement beam of        radiation such that an individually controllable element of an        array of individually controllable elements capable of        modulating a beam of radiation, is illuminated by the        measurement beam and redirects the measurement beam; and    -   a detector arranged to receive the redirected measurement beam        and determine the position at which the redirected measurement        beam is incident upon the detector, the position at which the        redirected measurement beam is incident upon the detector being        indicative of a characteristic of the individually controllable        element.

According to a further aspect of the invention, there is provided ameasurement method comprising:

-   -   illuminating, with a measurement beam of radiation, an        individually controllable element of an array of individually        controllable elements, the array being capable of modulating a        beam of radiation, the illuminating being such that the        individually controllable element redirects the measurement beam        of radiation;    -   receiving the redirected measurement beam at a detector; and    -   determining the position at which the redirected measurement        beam is incident upon the detector, the position at which the        redirected measurement beam is incident upon the detector being        indicative of a characteristic of the individually controllable        element.

According to an aspect of the invention, there is provided alithographic apparatus, comprising:

-   -   an illumination system arranged to provide an illumination beam        of radiation;    -   a measurement apparatus comprising:        -   a radiation source configured to provide a measurement beam            of radiation such that an individually controllable element            of an array of individually controllable elements capable of            modulating a beam of radiation, is illuminated by the            measurement beam and redirects the measurement beam, and        -   a detector arranged to receive the redirected measurement            beam and determine the position at which the redirected            measurement beam is incident upon the detector, the position            at which the redirected measurement beam is incident upon            the detector being indicative of a characteristic of the            individually controllable element.    -   a support structure configured to hold a patterning device, the        patterning device arranged to impart the illumination beam with        a pattern in its cross-section; and    -   a projection system arranged to project the patterned beam of        radiation onto a target portion of a substrate.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising:

-   -   providing an illumination beam of radiation using an        illumination system;    -   illuminating, with a measurement beam of radiation, an        individually controllable element of an array of individually        controllable elements, the array being capable of modulating a        beam of radiation, the illuminating being such that the        individually controllable element redirects the measurement beam        of radiation;    -   receiving the redirected measurement beam at a detector;    -   determining the position at which the redirected measurement        beam is incident upon the detector, the position at which the        redirected measurement beam is incident upon the detector being        indicative of a characteristic of the individually controllable        element;    -   using a patterning device to impart the illumination beam with a        pattern in its cross-section; and    -   projecting the patterned beam of radiation onto a target portion        of a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a measurement apparatus for an illuminator, whichcomprises part of the lithographic apparatus of FIG. 1;

FIG. 3 depicts an enlarged portion of the measurement apparatus of FIG.2;

FIGS. 4A to 4D depict portions of a detector array, which is part of themeasurement apparatus of FIG. 2;

FIG. 5 depicts a representation of the power intensity of a reflectedmeasurement beam of radiation, and the detection pattern of thereflected measurement beam at the detector array; and

FIG. 6 depicts a schematic representation of an illumination systemincorporating an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus incorporating ameasurement apparatus according to a particular embodiment of theinvention. The lithographic apparatus comprises:

-   -   an illumination system (illuminator) IL configured to provide a        beam PB of radiation (e.g. UV radiation);    -   a support structure (e.g. a mask table) MT configured to hold a        patterning device (e.g. a mask) MA and connected to a first        positioner PM configured to accurately position the patterning        device with respect to item PL;    -   a substrate table (e.g. a wafer table) WT configured to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate with respect to item PL; and    -   a projection system (e.g. a refractive projection lens) PL        configured to image a pattern imparted to the beam PB by the        patterning device MA onto a target portion C (e.g. comprising        one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

The support structure MT holds the patterning device in a way dependingon the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support can be using mechanical clamping, vacuum, or other clampingtechniques, for example electrostatic clamping under vacuum conditions.The support structure may be a frame or a table, for example, which maybe fixed or movable as required and which may ensure that the patterningdevice is at a desired position, for example with respect to theprojection system. Any use of the terms “reticle” or “mask” herein maybe considered synonymous with the more general term “patterning device”.

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise adjusting means AM for adjusting theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation PB, having a desired uniformity and intensity distribution inits cross-section.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the beam of radiation,and such components may also be referred to below, collectively orsingularly, as a “lens”.

In accordance with an embodiment of the invention, the illuminator ILfurther comprises a programmable mirror array 1 arranged to modulate thebeam PB, as will be described in more detail below. FIG. 1 schematicallyillustrates the illuminator IL as a linear structure with the beam fromthe source SO entering at one end and the beam PB exiting at the otherend. However, it will be appreciated that this need not necessarily bethe case. Indeed, in an embodiment of the invention incorporating areflective programmable mirror array 1 within the illuminator IL, thebeam PB may exit the illuminator along a beam path transverse to thebeam path of the beam from the source SO.

The beam PB is incident on the patterning device MA, which is held onthe support structure MT. Having traversed the patterning device MA, thebeam PB passes through the projection system PL, which focuses the beamonto a target portion C of the substrate W. With the aid of the secondpositioner PW and position sensor IF (e.g. an interferometric device),the substrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioner PM and another position sensor (which is not explicitlydepicted in FIG. 1) can be used to accurately position the patterningdevice MA with respect to the path of the beam PB, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe object tables MT and WT will be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the positioning means PM and PW.However, in the case of a stepper (as opposed to a scanner) the supportstructure MT may be connected to a short stroke actuator only, or may befixed. Patterning device MA and substrate W may be aligned usingpatterning device alignment marks M1, M2 and substrate alignment marksP1, P2.

The depicted apparatus can be used in the following preferred modes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to thebeam is projected onto a target portion C in one go (i.e. a singlestatic exposure). The substrate table WT is then shifted in the X and/orY direction so that a different target portion C can be exposed. In stepmode, the maximum size of the exposure field limits the size of thetarget portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the beam is projectedonto a target portion C (i.e. a single dynamic exposure). The velocityand direction of the substrate table WT relative to the supportstructure MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizes aprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As noted above, there may be provided a patterning device MA (e.g. anarray of individually controllable elements) that modulates the beam PBto create a pattern on the target portion of the substrate. Generally,the pattern created on the target portion of the substrate willcorrespond to a particular functional layer in a device being created inthe target portion, such as an integrated circuit or a flat paneldisplay (e.g., a color filter layer in a flat panel display or a thinfilm transistor layer in a flat panel display). Examples of patterningdevices include, e.g., masks, programmable mirror arrays, laser diodearrays, light emitting diode arrays, grating light valves, and LCDarrays. Patterning devices whose pattern is programmable with the aid ofelectronics (e.g., a computer), such as patterning devices comprising aplurality of programmable elements that can each modulate the intensityof a portion of the radiation beam, (e.g., all the devices mentioned inthe previous sentence except for the mask), including electronicallyprogrammable patterning devices having a plurality of programmableelements that impart a pattern to the radiation beam by modulating thephase of a portion of the radiation beam relative to adjacent portionsof the radiation beam, are collectively referred to herein as “contrastdevices”. In an embodiment, such a patterning device comprises at least10 programmable elements, e.g. at least 100, at least 1000, at least10000, at least 100000, at least 1000000, or at least 10000000programmable elements.

Embodiments of several of these devices are discussed in some moredetail below:

-   -   A programmable mirror array. This may comprise a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident radiation as diffracted        radiation, whereas unaddressed areas reflect incident radiation        as undiffracted radiation. Using an appropriate spatial filter,        the undiffracted radiation can be filtered out of the reflected        beam, leaving only the diffracted radiation to reach the        substrate; in this manner, the beam becomes patterned according        to the addressing pattern of the matrix-addressable surface. It        will be appreciated that, as an alternative, the filter may        filter out the diffracted radiation, leaving the undiffracted        radiation to reach the substrate. An array of diffractive        optical MEMS devices may also be used in a corresponding manner.        A diffractive optical MEMS device is comprised of a plurality of        reflective ribbons that may be deformed relative to one another        to form a grating that reflects incident radiation as diffracted        radiation. A further alternative embodiment of a programmable        mirror array employs a matrix arrangement of tiny mirrors, each        of which may be individually tilted about an axis by applying a        suitable localized electric field, or by employing piezoelectric        actuator. Once again, the mirrors are matrix-addressable, such        that addressed mirrors reflect an incoming radiation beam in a        different direction to unaddressed mirrors; in this manner, the        reflected beam may be patterned according to the addressing        pattern of the matrix-addressable mirrors. The required matrix        addressing may be performed using suitable electronics. More        information on mirror arrays as here referred to can be gleaned,        for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and        PCT patent applications WO 98/38597 and WO 98/33096, which are        incorporated herein by reference in their entirety.    -   A programmable LCD array. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference in its entirety.

The lithographic apparatus may comprise one or more patterning devices,e.g. one or more contrast devices. For example, it may have a pluralityof arrays of individually controllable elements, each controlledindependently of each other. In such an arrangement, some or all of thearrays of individually controllable elements may have a commonillumination system (or part of an illumination system), a commonsupport structure for the arrays of individually controllable elements,and/or a common projection system (or part of the projection system).

In accordance with an embodiment of the invention, the illuminator ILfurther comprises an array of individually controllable elements 1, suchas a programmable mirror array, arranged to modulate the illuminationbeam. The array of individually controllable elements selectively directportions of the illumination beam in order to provide a shapedillumination that can be controlled, and thus changed from one crosssectional pattern to another. That is, the array of individuallycontrollable elements is arranged to modulate the pupil shape of theillumination beam.

In an embodiment, the array of individually controllable elements 1comprises a programmable mirror array of individually controllablemirror elements within the illuminator. Such a programmable mirror arraymay be similar to a programmable mirror array used as a patterningdevice to impart the pattern to the beam to be projected onto a targetportion of the substrate as described above for the lithographicapparatus of FIG. 1. The skilled person will appreciate that alternativepatterning devices that are known for use within a lithographicapparatus may be suitable for use within the illuminator. However, thenumber of individually controllable elements within the illuminator istypically fewer. For instance, the array of individually controllableelements within the illuminator may comprise approximately 60×60individually controllable elements. Furthermore, each individuallycontrollable element within the illuminator is typically arranged suchthat it can be tilted in two orthogonal directions, whereas within thepatterning device each individually controllable element typically onlytilts in a single direction.

In a programmable mirror array embodiment, control of the tilt angle foreach mirror element is achieved by control of charged plates positionedbehind each mirror element. Each mirror element is electro-staticallyattracted to or repelled by the charged plates. Each mirror element istypically of the order of 0.8 mm×0.8 mm, and may be tilted byapproximately plus or minus 5° from its center position. The requiredaccuracy for the tilt of an individual mirror element is approximately1/1000 of the full-scale movement, (or 0.01° for full-scale movement of10°). Each time the position of a mirror is altered the settling time isapproximately 10 ms.

Being able to modulate the illumination beam that is incident upon thearray of individually controllable elements may be desirable for anembodiment of a lithographic apparatus in which it is desirable to beable to rapidly switch between different cross sections of anillumination beam. Additionally or alternatively, such a controllablearray may be advantageous in that it is relatively cheap and flexible inproviding any desired illumination setting. For instance, it may benecessary to switch between different lithographic patterning devices inorder to project patterns onto different sections of the substrate. Eachpatterning device may itself require an illumination beam with adifferent illumination mode (i.e. angular distribution). As noted above,a lithographic apparatus may provide such varying modes for theillumination beam by providing a mask in the illuminator that can bechanged between exposures of the substrate. However, it can be timeconsuming to change the illuminator mask, for instance when thepatterning device is switched. Therefore, the ability to rapidly andcontrollably change the cross section of the illumination beam bycontrolling an array of individually controllable elements may beadvantageous.

However, the unmodulated illumination beam incident upon the array ofindividually controllable elements is typically at a relatively highintensity, such that the array of individually controllable elements canbecome heated. Such heating can affect the accuracy with which theillumination beam is selectively directed, thus causing the shapedillumination beam to deviate from its expected shape.

For a patterning device comprising a programmable mirror array, theintensity of the beam is reduced due to the number of optical elementsthrough which the beam has passed before reaching the patterning device.This means that the individual mirrors are not heated as much, and thusthe accuracy of the mirror array is higher. Consequently, for thepatterning device, it is typically not necessary to measure the tiltangle of each mirror element during a lithographic exposure operation.Acceptable levels of accuracy may be achievable using measurementsystems positioned on the substrate table in between lithographicoperations.

To achieve an acceptable level of accuracy for an array of individuallycontrollable elements within the illuminator, it may be necessaryhowever to measure the tilt angle of individual elements during alithographic operation. This is due to changes in the materialproperties of individual elements as a result of the thermal loadcausing the tilt angle of elements to tilt unpredictably. The substrateupon which the array of individually controllable elements is mountedmay also be heated causing the elements to tilt unpredictably. Inaddition or alternatively to the effects of heating of the elements andthe substrate, other factors may cause the tilt of individual elementsto change over time, causing a reduction in the tilt accuracy ofelements. For instance, mechanical deformation of the substrate causedby reaction forces due to element actuation may cause the tilt tochange.

In an embodiment, the array of individually controllable elementscomprises mirror elements mounted upon control electronics arranged tocontrol the tilt of each mirror element. The available volume behindeach mirror element is limited, and thus it is desirable not to locateone or more tilt sensors behind the mirror elements. Furthermore, thetemperature in the direct area around the mirror elements is subject tosignificant variation, which would be liable to affect the one or moretilt sensors. Further, it is not desirable to mount one or more tiltsensors on the mirror side of each element, as this would reduce theavailable reflective area.

In accordance with an embodiment of the invention, there is provided anoptical measurement method and apparatus capable of determining the tiltangle of individually controllable elements of the array of individuallycontrollable elements in the illuminator in real time duringlithographic operations, without interfering with the lithographicoperation. Such an optical approach is advantageous in that it can beplaced at a relatively large distance from the array of individuallycontrollable elements and thus is not itself adversely affected by thetemperature instability of the array. Additionally, as the opticalsystem does not contact the array, it does not directly affect the tiltof the elements.

As the optical system can be used in real time, the measurement of thetilt of an individual element can be fed back to the lithographicapparatus and used to alter the control signal provided to theprogrammable array within the illuminator in order to correct the tiltof individual elements which are found to have deviated from theirexpected tilt angle.

Referring to FIG. 2, there is schematically illustrated a measurementsystem that is part of the illuminator IL of the lithographic apparatusof FIG. 1 in accordance with an embodiment of the invention.Programmable mirror array 1 is depicted reflecting the beam from thesource SO towards the patterning device MA, schematically represented byline 2. The other components of the illuminator IL are not shown in FIG.2 for reasons of clarity.

In order to measure the tilt of individual mirror elements, a pluralityof separate radiation sources 3 are provided, each of which provides aseparate measurement beam of radiation 4. The radiation sources 3 maycomprise LEDs or lasers, depending upon the sensitivity or thesaturation level of a detector 5 (described below). Each measurementbeam is focused by a respective lens 6 (or a suitable micro array oflenses) such that the measurement beams 4 pass through respectiveapertures 7. In an embodiment, a single radiation source 4 could provideeach of the measurement beams 4.

The emergent beams from apertures 7 are then focused by lenses 8 suchthat each measurement beam 4 is incident upon a respective mirrorelement within the programmable mirror array 1. That is, each mirrorelement is illuminated by a spot from a separate measurement beam 4. Thealignment of each radiation source 3 and/or radiation beam 4 with therespective mirror element of the programmable mirror array 1 is notcritical, so long as the spot size at each mirror element is smallcompared to the reflective area of that mirror element. Each mirrorelement reflects the measurement beam 4 as a reflected measurement beam9. The angle of incidence of the measurement beams 4 upon theprogrammable mirror array 1 is chosen such that the reflectedmeasurement beams 9 are reflected away from radiation sources 3 towardsa detector 5. The reflected measurement beams 9 are focused towards thedetector 5 by a lens 10. In an embodiment, multiple detectors 5 may beprovided.

The detector 5 comprises an image sensor. The image sensor may comprisea detector array such as a charge-coupled device (CCD) comprising anarray of individual radiation detectors or pixels. The position at whicheach beam is incident upon the detector array 5 is measured. Relativemovement of a reflected measurement beam 9 is detected by the detectorarray 5. Changes in the tilt angle of a mirror element cause movement ofthe point at which the reflected measurement beam 9 is incident upon thedetector array 5. Thus, measurement of the position of the measurementbeam 9 upon the detector array provides a determination of the tiltangle of the mirror element.

The measurement system is separate from optical elements within theilluminator configured to generate and focus the illumination beam.Indeed, it can be seen from FIG. 2 that the measurement beams 4 and thereflected measurement beams 9 travel along beam paths separate from thepath 2 of the beam to be patterned by the patterning device MA.Furthermore, in order to prevent interference with that beam, themeasurement apparatus makes use of one or more radiation sources 3 whichemit radiation at a wavelength different to that of the beam, forinstance within the green to red portion of the visible spectrum.

Referring to FIG. 3, an enlargement of part of FIG. 2, indicated by thedotted circle 11, is schematically illustrated. FIG. 3 shows tworeflected measurement beams 9 and the position of the detector array 5.It can be seen that the detector array 5 is outside of the focal plane12 of the reflected measurement beams. This is in order to detectmovement of the reflected measurement beams 9 across the detector. Ifthe detector array 5 were to be in the focal plane, an image of themirror array would be received, and movement of the measurement beamswould not be detected. The detector array 5 may be positioned either infront of the focal plane 12 (as illustrated in FIG. 3) or behind thefocal plane 12. Furthermore, by positioning the detector array 5 out ofthe focal plane 12, each reflected measurement beam 9 illuminates thedetector array 5 with a broad spot, rather than a point. It is desirablefor each reflected measurement beam 9 to illuminate more than one cellof the detector array 5 in order for movement of the position of thereflected measurement beam 9 to be detected with a greater level ofaccuracy (as will be described in greater detail below with reference toFIGS. 4A to 4D).

Using a commercially available two dimensional detector array, it ispossible to measure the tilt of an individual mirror element in two tiltdirections to an accuracy of 1/1000 of the full-scale tilt of eachmirror element, at a measurement rate of up to 5 kHz. This allows forhigh-speed tilt error correction. As the update rate of the controlelectronics that control each mirror element is approximately 200-400 Hzfor a commercially available mirror array, it is clear that themeasurement apparatus is sufficiently fast to respond before a nextcontrol signal is sent to the mirror element. The tilt angle of a singlemirror element can be measured. Alternatively or additionally, the tiltangles of a group of mirror elements, or the whole of the mirror array,can be measured.

For each mirror element, the position at which the reflected measurementbeam 9 intersects the detector array 5 when the mirror element is at adefault position (for instance no tilt) is determined. This can beestablished by calibrating the optical system of FIG. 2 using anindependent measurement system, for instance one positioned on thesubstrate table, when the lithographic apparatus is not projecting ontoa substrate. The independent measurement system is used to steer eachmirror element into a no tilt position, and then the position of thereflected measurement beam within the detector array is measured. Thetilt angle is then varied and the corresponding variation in spotposition at the detector array recorded. After the system has beencalibrated in this manner then variation in the tilt of each mirrorelement can be measured while the lithographic apparatus is projectingonto a substrate, and if necessary used to alter the control signalprovided to the mirror element to correct the tilt angle. Thiscalibration process may be repeated periodically in order to ensure theaccuracy of the measurement system over time.

The detector array 5 may comprise a two-dimensional image system, suchas a CCD. A commercially available image system that may be suitableoffers resolution of 1024×1024 pixels, each pixel approximately 12 μm×12μm. It can be seen that the number of available pixels is much greaterthan the number of reflected measurement beams projected onto thedetector array.

In order to reduce the number of outputs from the CCD, in accordancewith an embodiment of the invention, groups of pixels are combined suchthat the combined signal from the group is provided as a single output.FIG. 4A schematically illustrates a group of four pixels 20 combined toform a single detector group 21. Each pixel 20 measures 12 μm×12 μm,such that the detector group 21 measures 24 μm×24 μm. Combining groupsof pixels in this way has an advantage of speeding up the data outputfrom the CCD.

As noted above, it is desirable that the detector array 5 is positionedso that the reflected measurement beam 9 is larger than a single pixel20 (or group of pixels 21 if they are combined as described above). Thisincreases the measurement accuracy as the projected spot moves acrossthe detector array 5. The movement of a spot is detected as theproportion of radiation detected by each group 21 varies. FIG. 4Bdepicts four groups of pixels 21A-21B illuminated by a spot 22 from asingle reflected measurement beam 9. It can be seen that spot 22 isincident equally upon all four groups 21A-21D. Consequently, theproportion of the incident radiation detected by each group of pixelswill be the same. As long as the proportion of radiation detected byeach group of pixels is known for the no tilt position, any deviationfrom this known detection pattern can be detected and the correspondingchange in tilt angle determined.

FIG. 4C illustrates a situation in which the spot 22 has moved in afirst direction, due to the mirror element tilting about a first axis.It can be seen that the proportion of the incident radiation detected bypixel groups 21B and 21D will have increased, whereas the proportion ofthe incident radiation detected by pixel groups 21A and 21C will havedecreased. From the change in the proportion of radiation detected byeach pixel group, given knowledge of the intensity profile of thereflected measurement beams, the distance by which the incident spot hasmoved can be determined, and consequently the change in tilt angle forthe associated mirror element determined. Similarly, FIG. 4D illustratesthe situation in which the mirror element tilt angle has changed about adifferent axis. It will be appreciated that if the tilt angle haschanged about both axes then this will also be detectable.

It will be appreciated that the size of the beam spot may be varied, forinstance by changing the position of the detector array 5 relative tothe focal plane 12 of the reflected measurement beams or by adjustingany one or more of lenses 6, 8 and 10. Consequently, the number ofpixels or groups of pixels illuminated by each beam may change. In theexample embodiment of FIGS. 4B to 4D, sixteen pixels 20 (in four groups21 of four pixels) are reserved for detecting the position of a singlereflected measurement beam 9. The spot size is chosen so that it willnot stray outside of the area of the CCD reserved for detecting a singlereflected measurement beam when the associated mirror element is at fulltilt. This is to ensure that none of the radiation from a singlereflected calibration beam is incident upon areas of the detector arrayreserved for detecting other reflected measurement beams.

The intensity profile across the beam is chosen so as to optimize thedetection of a shift in the position of the spot. As the tilt of themirrors is related to the displacement of the spot on the detectorarray, it is desirable to have a beam intensity profile thatcontinuously decreases in intensity from the center of the beam, forinstance a Gaussian-like intensity profile, in order to determine themirror tilt. FIG. 5 depicts a suitable intensity profile 30 for areflected measurement beam 9 as detected at the detector array 5. It canbe seen that the intensity profile 30 rapidly reduces to zero at theextremes of the profile, which as noted above is desirable to preventinterference between reflected measurement beams of radiation acrossdifferent portions of the detector array. The beam intensity profile isdetermined by properties of one or more of the lenses 6, 8, 10 and theirseparation.

As noted above, in order to increase the rate at which data can beoutput from the detector array 5, it is desirable to combine the outputsof groups of pixels 21. The minimum number of pixels 20 or groups ofpixels 21 needed in order to determine the center position of a spotmoving along a single axis is two. Referring to FIG. 5, top line 31illustrates two groups of pixels 32 used to detect a spot projected by abeam having the radiation intensity profile 30. It can readily be seenthat when the spot moves along axis 33, the proportion of radiation(shown by the shading—the darker, the higher the proportion) detected bypixel groups 32 will vary. The signal ratio of the outputs from pixelgroups 32 gives a measure of the position of the spot along axis 33.Alternatively, a larger number of pixel groups may be illuminated by asingle spot (either by increasing the size of the spot, or reducing thenumber of pixels per group). Line 34 depicts 16 groups of pixels 35,which are arranged to detect the spot projected by the beam having theradiation intensity profile 30, the proportion of radiation detected byeach pixel group 35 shown by its shading—the darker, the higher theproportion.

Referring to FIG. 6, an illuminator for a lithographic apparatusincorporating a measurement system in accordance with an embodiment ofthe invention is schematically illustrated. The illuminator is arrangedto pass a beam along beam path 40 from the beam source to the patterningdevice. Beam path 40 intersects the programmable mirror array 1, whichselectively reflects portions of the beam. The measurement radiationsources 3 are collectively depicted as a single source 41. Detectorarray 5 is schematically illustrated as a camera 42, with the radiationreflected between the source 41 and the camera 42 schematicallyindicated by beams 4 and 9. FIG. 6 shows that the measurement beam 4 andthe reflected measurement beam 9 do not interfere with the beam path 40of the beam. Other portions of the illuminator of FIG. 6 areconventional, and thus will not be described in detail here.

It will be appreciated that the above described measurement system foran array of individually controllable elements within an illuminator maybe applied in both reflective and transmissive lithographic apparatuses.

It will be appreciated that while one or more embodiments of theinvention have been predominantly described as being arranged to measurethe tilt angle of mirror elements forming a programmable mirror arraywithin an illuminator, it is not limited to this. Indeed, a tilt anglemeasurement apparatus in accordance with an embodiment of the inventionmay be applied to any array of individually controllable elements, forinstance a micro mirror array forming the patterning device for alithographic apparatus or grating light valves forming the array ofindividually controllable elements.

As used herein, the term redirected or redirecting broadly includes,respectively, reflected or reflecting, refracted or refracting,diffracted or diffracting, etc.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the measurement apparatus and lithographic apparatusdescribed herein may have other applications, such as the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist) or a metrology or inspection tool.Where applicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm), as well as particle beams, such as ion beams or electronbeams.

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a beam with apattern in its cross-section such as to create a pattern in a targetportion of the substrate. It should be noted that the pattern impartedto the beam may not exactly correspond to the desired pattern in thetarget portion of the substrate. Generally, the pattern imparted to thebeam will correspond to a particular functional layer in a device beingcreated in the target portion, such as an integrated circuit.

A patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “projection lens” herein may beconsidered as synonymous with the more general term “projection system”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables (and/or supportstructures) may be used in parallel, or preparatory steps may be carriedout on one or more tables (and/or support structures) while one or moreother tables (and/or support structures) are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A measurement apparatus comprising: a radiation output configured toprovide a measurement beam of radiation such that an individuallycontrollable element of an array of individually controllable elementscapable of modulating a beam of radiation, is illuminated by themeasurement beam and redirects the measurement beam; and a detectorarranged to receive the redirected measurement beam and determine tiltof the individually controllable element from the redirected measurementbeam.
 2. The measurement apparatus according to claim 1, wherein thedetector is positioned out of a focal plane of the redirectedmeasurement beam.
 3. The measurement apparatus according to claim 1,wherein the detector comprises an image sensor comprising an array ofpixel elements, each pixel element arranged to provide an output signalindicative of received radiation.
 4. The measurement apparatus accordingto claim 3, wherein the outputs of groups of pixel elements are groupedand the redirected measurement beam illuminates at least two pixelelements or groups of pixel elements.
 5. The measurement apparatusaccording to claim 4, wherein the detector is arranged to measure aproportion of radiation received by each pixel element or group of pixelelements from the redirected measurement beam in order to determine aposition at which the redirected measurement beam is incident upon thedetector.
 6. The measurement apparatus according to claim 1, furthercomprising a separate measurement system arranged to receive the beam ofradiation and determine tilt of the individually controllable element inorder to calibrate the determination of tilt of the individuallycontrollable element by the detector.
 7. The measurement apparatusaccording to claim 1, further comprising a plurality of radiationsources configured to provide a plurality of measurement beams ofradiation and arranged such that a plurality of individuallycontrollable elements are illuminated with separate measurement beamsand redirect a plurality of respective redirected measurement beams, thedetector being arranged to receive the redirected measurement beams anddetermine tilt of each of the plurality of individually controllableelements from the redirected measurement beams.
 8. A lithographicapparatus, comprising: an array of individually controllable elementscapable of modulating a beam of radiation; a measurement apparatuscomprising: a radiation output configured to provide a measurement beamof radiation to an individually controllable element of the array ofindividually controllable elements, and a detector arranged to receivethe measurement beam as redirected by the individually controllableelement and to determine tilt of the individually controllable elementfrom the redirected measurement beam; and a projection system arrangedto project the modulated beam of radiation onto a radiation-sensitivetarget portion of a substrate.
 9. The apparatus according to claim 8,wherein the array of individually controllable elements comprises aprogrammable mirror array comprising a plurality of mirror elements eacharranged to tilt in order to alter the direction in which they reflectthe beam of radiation and the measurement beam.
 10. The apparatusaccording to claim 8, wherein the measurement apparatus is furtherarranged to use the determined tilt to alter a control signal providedto the individually controllable element in order to correct tilt of theindividually controllable element.
 11. The apparatus according to claim8, wherein the measurement beam and the redirected measurement beamtravel along beam paths that are not coincident with the beam path ofthe beam of radiation.
 12. A method comprising: illuminating, with ameasurement beam of radiation, an individually controllable element ofan array of individually controllable elements, the array being capableof modulating a beam of radiation, the illuminating being such that theindividually controllable element redirects the measurement beam ofradiation; receiving the redirected measurement beam at a detector; anddetermining tilt of the individually controllable element from theredirected measurement beam.
 13. The method according to claim 12,comprising receiving the redirected measurement at the detector out of afocal plane of the redirected measurement beam.
 14. The method accordingto claim 12, wherein the detector comprises an image sensor comprisingan array of pixel elements, each pixel element arranged to provide anoutput signal indicative of received radiation.
 15. The method accordingto claim 14, wherein the outputs of groups of pixel elements are groupedand comprising illuminating at least two pixel elements or groups ofpixel elements with the redirected measurement beam.
 16. The methodaccording to claim 15, comprising measuring a proportion of radiationreceived by each pixel element or group of pixel elements from theredirected measurement beam in order to determine a position at whichthe redirected measurement beam is incident upon the detector.
 17. Themethod according to claim 12, further comprising calibrating thedetermination of tilt of the individually controllable element by thedetector using a separate measurement system arranged to receive thebeam of radiation and determine tilt of the individually controllableelement.
 18. The method according to claim 12, further comprising usingthe determined tilt to alter a control signal provided to theindividually controllable element in order to correct the tilt of theindividually controllable element.
 19. The method according to claim 12,wherein the measurement beam and the redirected measurement beam travelalong beam paths that are not coincident with the beam path of the beamof radiation.
 20. The method according to claim 12, further comprising:modulating the beam of radiation using the array of individuallycontrollable elements; and projecting the modulated beam of radiationonto a radiation-sensitive target portion of a substrate.